Various means have been proposed heretofore for reducing and suppressing the noise in the exhaust duct of turbine engines, air conditioning duct systems, and similar equipment. The most common sound absorption technique of the prior art is the use of an absorptive lining within the duct as shown, for example, in U.S. Pat. No. 3,542,152, granted to Arthur P. Adams et al. It is also well known in the art to reduce the noise generated by appurtenances coming into contact with the air stream, such as flow splitters, guide vanes, fan blading, and the like, by reducing the wake emanating thereby through the utilization of boundary layer control. A prior art reference disclosing this approach is the literature publication, "Quiet Engine Nacelle Design," M. Dean Nelson, NASA SP-311, Aircraft Engine Noise Reduction, 1972. Other flow-duct noise abating means are shown in U.S. Pat. No. 3,503,495 to Kobayashi et al, U.S. Pat. No. 3,511,336 to Rink et al, U.S. Pat. No. 3,820,638 to Hanson. Still another approach to noise reduction is that suggested in U.S. Pat. No. 3,033,494 to Tyler et al, wherein noise reduction of jet engine exhaust may be provided by means of shaping the jet engine exhaust duct. The use of wedge-shaped bodies for the dissipation of high frequency vibratory energy is disclosed in U.S. Pat. No. 3,058,015 to Nesh.
A practical limitation of all of the aforementioned prior art devices is the limited frequency spectrum over which they can effectively function. There exists a need for a broadband sound absorber of comparable efficiency. Classical acoustic theory indicates that for any given duct there is a given acoustic impedance spectrum of the duct liner which would provide the best possible attenuation at all frequencies. However, there are no known practical materials capable of providing an acoustic impedance which is the optimum function of frequency. Resistance which increases with frequency has been attained, but a reactance which becomes more negative as the frequency increases has not been attained in any practical way. Thus, the attenuation attained in real ducts falls far short of theoretically attainable values, except for a single frequency.